1. Field of Invention
The present invention relates to a frequency shift keying (FSK) modulator. More particularly, the present invention relates to a voltage controlled oscillator (VCO) suitable for use in a FSK system.
2. Description of Related Art
An oscillator is an electric or electronic circuit that produces an output signal that oscillates at a principal oscillation frequency and, in some instances, contains harmonic frequency signals (i.e., signals having frequencies that are multiples of the principal oscillation frequency). Some oscillators have a principal oscillation frequency that may be varied by the application of a suitable control voltage. Oscillators of this type, called voltage-controlled oscillators or VCO's, are used in many communication and other signal-processing applications. Because suppression or reduction of higher-order harmonics of the principal oscillation frequency and other noise in the output signal of a VCO is often desirable.
In a VCO for switching oscillation frequency bands by supplying a band switching signal to an LC resonator circuit that is connected in a phase locked loop having a phase comparator and a loop filter and sets oscillation frequencies, the LC resonator circuit includes at least one inductor and one capacitor, the connection and non-connection of a part of which are switched in response to a band switching signal and a capacitor element. Wherein, a voltage variable capacitor, or called varactor, element whose capacitance value is adjusted in response to the control voltage from the loop filter is connected in series to impedance elements acting as a composite element that exhibits a capacitance value in oscillation frequency ranges. With this arrangement, when an oscillation signal is selectively output in a high or low frequency band by switching the LC resonator circuit, the voltage controlled oscillator can increase the difference between the rates of change in the high and low frequency bands.
In communication system, the VCO is implemented in a FSK modulator. FIG. 1 is a block diagram, schematically illustrating the conventional FSK modulator. In FIG. 1, a conventional FSK modulator includes a VCO 20, a frequency divider 22, a low pass filter (LF) 24, and a frequency mixer 26. The VCO 20 further includes an oscillator for producing a fosc ∝1/√{square root over (LC)}, where C contains all related capacitance at VCO output. The frequency divider 22 is used for dividing the oscillation output. The mixer 26 is used for synthesizing frequency of the divided output with that of a reference wave signal. The LF 24 is used for removing high-frequency components of this output. The input data (DATA) is, for example, a no-return-zero (NRZ). In addition, it is assumed that a data row of the input data is temporarily at a 0 or 1 in voltage signal level.
In wireless communication, the digital binary data is carried by a central frequency f0 for transmission. FIG. 2 is a drawing, schematically the typical frequency spectrum of FSK. In order to represent the digital value 0 and digital value 1, a positive frequency shift +fm and a negative frequency shift −fm are used. In the configuration of FIG. 1, the frequency of the reference signal which is an output of a reference signal generator which is not shown is synthesized with that of a divided output of the frequency divider 22 by means of the mixer 26 so as to output a desired frequency f0. The VCO 20 detects an output of the mixer 26 and converts it to a voltage so as to synthesize it with a voltage level (0 [V] or 1 [V]) of a data row. For example, an output frequency of the VCO 20 becomes a frequency f1 (f1=f0−fm1) when a voltage level of a data row is at level 0 [V] and it becomes a frequency f4 (f2=f0+fm4) when it is at level 1 [V], so that there is a difference of fm1+fm4 between these frequencies. As described above, the FSK modulation is achieved by shifting a frequency from the intermediate frequency f0 by ±fm. Since the actual frequency being used is not a fixed value, and can be varied. Therefore, various frequency shifts are necessary to be set, in order to have the optimal frequency to transmit data without errors.
Conventionally, the frequency fi can be selected by setting different voltage Vi. The desired frequency spacing between digital level 0 [V] and 1 [V] can be obtained by selecting a pair of frequency [fi, fj] with the desired frequency spacing.
However, in the conventional FSK modulator, the frequency spacing is fixed. There is no way to further adjust the frequency spacing to get more precise frequency spacing. If the frequency is more precise, the data error is less. Therefore, it is an issue about how to get more precise frequency spacing to represent the binary data.